Photoflash arrays normally include a plurality of flashlamps affixed to a circuit board. A printed circuit on the circuit board is electrically connected to the flashlamps and to a source of potential. The potential or energizing source may be of the so-called high voltage type wherein a voltage in the range of about 2000 volts is available or of the low voltage type wherein the voltage provided is in the range of about 3 to 15 volts. Also, the array normally includes a plurality of radiation-responsive switches positioned adjacent the flashlamps and electrically connected into the printed circuit. Often these radiation-responsive switches are of the normally open (N/O) type and respond to radiation from a nearby flashlamp to provide an electrically conductive path.
One of the problems encountered in photoflash arrays and particularly in arrays having severely restricted space limitation on the distances separating the components of the array is a tendency toward defects due to humidity. When the arrays, which normally include radiation-responsive switches, are subjected to humid or tropical conditions wherein moisture condensation is encountered, the moisture tends to undesirably react with the switch material. Unfortunately, the reaction to this undesired moisture is to provide a conductive path or defective array since the conductive path is in response to moisture rather than radiation from a flashlamp. Thus, the trend toward arrays of smaller size and increased components undesirably decreases the spacing and increases the susceptibility to defects due to humidity.
One known attempt to reduce the effects of humid conditions on flashlamp arrays and particularly those which include convertible switches is suggested in U.S. Pat. No. 4,136,376, issued Jan. 29, 1979. Therein, a photoflash array includes a plurality of switches responsive to a relatively high voltage source. A coating of electrically insulative adhesive is applied to and covers the switches in order to prevent short circuits due to moisture. However, it appears that such electrical adhesive responsive to relatively high voltages is an acceptable material only so long as openings are provided for electrical termination and the coating only covers the switch devices.
Another known suggestion for reducing the effects of humidity on a switch is set forth in U.S. Pat. No. 3,969,065, issued July 13, 1976. Therein, a cupric oxide material is employed in conjunction with an organic binder. The conversion of the cupric oxide to copper with the accompanying combustion of the organic binder improved the resistance of the array to high humidity. Thus, a copper salt activity is relied upon for humidity control.
Still another technique for controlling the effects of humidity on a switch arrangement is set forth in U.S. Pat. No. 3,951,582, issued Apr. 20, 1976.
Therein, a light absorbing heat transmitting coloring agent and a resin and solvent are utilized to provide a protective switch coating. Moreover, a U.S. patent application bearing U.S. Ser. No. 332,899, filed Dec. 17, 1981 and assigned to the Assignee of the present application, suggests a humidity-protective system which includes titanium dioxide and a binder of blended polystyrene and a thermoplastic.
Although each of the above-mentioned suggestions and materials have provided protection against humid conditions, it has been found that there is something still to be desired insofar as moisture barriers for arrays having relatively closely spaced components are concerned. Moreover, arrays responsive to relatively low voltage sources wherein switch conversion necessitates especially low resitance values are particularly susceptible to humid conditions.